The entire reason the "tap" is there is to alert nage/tori/instructor/whatever that uke's boundaries have been reached. Some tap right when they reach it, some tap on a hard limit. Still, it's a safety measure ... for training. Safety measures are there for a reason.

Mistakes can be made in either direction. If uke taps too soon, gets in the habit of tapping too soon, then they're not learning good ukemi. Never mind considerations of toughness or "developing a pain tolerance" (insofar as such a thing can be done) - whenever uke bails early, they're failing to take all the opportunities they may have in the situation. Maybe there's an opportunity to reverse. Or maybe there's an opportunity for a better, safer ukemi. If uke taps the second they're touched, they'll never develop those skills, and in a situation where their attacker doesn't care about tapping, they'll be in big trouble.

If nage fails to respect the tap and places uke's judgment subordinate to his/her own, they risk the possibility that their judgment is in error. If it is, they will do damage to uke's body, uke's trust, and their own authority, whatever that may be.

People will disagree about which of these mistakes is potentially more serious. Certainly you can argue that someone without good ukemi skills is risking more than a sore shoulder. But here's where it becomes crystal clear for me: no matter how big a mistake it may be to tap too soon, it is uke's mistake to make. It harms no one else. If you, as sensei, decide that you can't reason this person out of their bad habits, ease them through it, and continue to teach them, then it's your prerogative to no longer have them as a student. That, you can do. The mistake of misjudging how much your partner can take when they have clearly signaled that a limit has been reached, is not yours to make. No matter who you are, you are out of line when subordinate your partner's tap-out to your own judgment about what they can take.